Like any other organ or tissue structure, the heart requires oxygen and includes coronary arteries to facilitate the delivery of oxygenated blood to cardiac tissue. Unlike other organs, a disruption of the oxygen supplied by the coronary arteries often results in discernable electrical signals. These electrical signals may indicate ischemia or infarct and may also indicate the relative severity of the event.
A standard, 12-lead surface ECG (electrocardiogram) records electrical signals across multiple vectors and produces a highly accurate representation of cardiac data. Each cardiac cycle is distinctly represented. A P-wave is indicative of atrial depolarization, a QRS complex is indicative of ventricular depolarization and a T-wave is indicative of ventricular repolarization. The portion of the signal between the QRS complex and the T-wave is referred to as the ST segment. An elevation of the ST segment from a relative baseline is typically indicative of a complete and sudden coronary occlusion whereas a depression of the ST segment is indicative of another form of ischemia, such as demand ischemia. For example, a coronary artery may be partially occluded, e.g., due to stenosis, allowing sufficient blood flow under normal physiological conditions. When physiological demand increases, such as during exercise, the cardiac tissue is unable to receive sufficient oxygen through the partially occluded arteries, becoming ischemic. Typically, cessation of the activity reduces demand, and as the supply becomes sufficient the ischemia resolves, which resolution is indicated by the ST segment returning to the baseline value.
As indicated, the surface ECG provides accurate and detailed information. In addition, the data is often redundant as each channel that is recorded represents the same events as they occur over different vectors. A physician can therefore check multiple channels when evaluating the data for increased reliability and accuracy.
Implantable medical devices (IMD) often include sensors, e.g., implanted electrodes, that detect electrical cardiac signals. When collected internally, as opposed to on the surface (i.e., ECG), these signals are referred to as an electrogram (EGM). For some vectors, the cathode and anode of a given sensing pair may be relatively close together. For example, a tip electrode and a ring electrode on a common cardiac lead, referred to as a bipolar vector, sense electrical signals across a small portion of the heart. The device housing may include one or more electrodes. Thus, sensing from a unipolar vector, e.g., a tip electrode or coil electrode to the housing (“can”) electrode, provides a vector across a greater portion of the heart. These examples relate to implantable pulse generators (IPGs), often referred to as pacemakers or low power devices and implantable cardioverter/defibrillators (ICDs), often referred to as high powered devices, which may also include pacing functionality. IPGs and ICDs typically include a housing implanted subcutaneously or submuscularly and connected to one or more leads that transvenously enter the heart. Other devices, such as an implantable loop recorder (ILR) are implanted subcutaneously to record data, but do not include leads extending to or into the heart.